Chemi-mechanical polishing of semiconductor wafers is useful, at various stages of device fabrication, for planarizing irregular top surface topography, inter alia. For example, in the process of fabricating modern semiconductor integrated circuits, it is necessary to form conductive lines or other structures above previously formed structures. However, prior structure formation often leaves the top surface topography of the silicon wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches and/or other surface irregularities. As a result of these irregularities, deposition of subsequent layers of materials could easily result in incomplete coverage, breaks in the deposited material, voids, etc., if it were deposited directly over the aforementioned highly irregular surfaces. If the irregularities are not alleviated at each major processing step, the top surface topography of the surface irregularities will tend to become even more irregular, causing further problems as layers stack up in further processing of the semiconductor structure.
Depending upon the type of materials used and their intended purposes, numerous undesirable characteristics are produced when these deposition irregularities occur. Incomplete coverage of an insulating oxide layer can lead to short circuits between metallization layers. Voids can trap air or processing gases, either contaminating further processing steps or simply lowering overall device reliability. Sharp points on conductors can result in unusual, undesirable field effects. In general, processing high density circuits over highly irregular structures can lead to very poor yield and/or device performance.
Consequently, it is desirable to effect some type of planarization, or flattening, of integrated circuit structures in order to facilitate the processing of multi-layer integrated circuits and to improve their yield, performance, and reliability. In fact, all of today's high-density integrated circuit fabrication techniques make use of some method of forming planarized structures at critical points in the fabrication process.
Planarization techniques generally fall into one of several categories: chemical/mechanical polishing techniques; leveling with a filler material then etching back in a controlled environment; and various reflow techniques. Etching techniques can include wet etching, dry or plasma etching, electro-polishing, and ion milling, among others. A few less common planarization techniques exist, such as direct deposition of material into a trench by condensing material from a gaseous phase in the presence of laser light. Most of the differences between modern planarization techniques exist in the points in processing that the different techniques are applied, and in which methods and materials are used.
The present invention is directed to chemi-mechanical polishing, which generally involves rubbing a wafer with a polishing pad in a slurry containing both an abrasive and chemicals. Typical slurry chemistry is KOH (Potassium Hydroxide), having a pH of about 11. Generally, polishing slurry is expensive, and cannot be recovered or reused. Typical usage (feed) rates for slurry are on the order of 175 ml (milli-liters) per minute. A typical silica-based slurry is "SC-1" available from Cabot Industries. Another, more expensive slurry based on silica and cerium (oxide) is Rodel "WS-2000".
Chemi-mechanical polishing is described in U.S. Pat. Nos. 4,671,851, 4,910,155, 4,944,836, all of which patents are incorporated by reference herein. When chemi-mechanical polishing is referred to hereinafter, it should be understood to be performed with a suitable slurry, such as Cabot SC-1.
The current state of the art in dielectric film polishing for silicon wafers suggests the use of more than one polishing pad. For example, two pads are secured into a "stack" which may be termed a "composite polishing pad". The top pad, which performs polishing, is typically stiffer than the more compliant bottom pad, which is mounted to a rotating platen. A pressure sensitive adhesive is typically used to adhere the pads together and to the platen.
FIG. 1 shows a typical technique for chemi-mechanical polishing. A first disc-shaped pad 102 (PAD A) having a layer of pressure sensitive adhesive 104 on its back face is adhered (shown exploded) to the front face of a rotating platen 106 (PLATEN). A second disc-shaped pad 108 (PAD B) having a layer of pressure sensitive adhesive 110 on its back face is adhered (shown exploded) to the front face of the first pad 102. The platen 106 is rotated, and a metered stream of slurry 112 (shown as dots) from a slurry supply 114 is delivered via a slurry feed 116 to the front face of PAD B.
Evidently, centrifugal forces and gravity will cause the slurry to flow (wash) over the periphery of PAD A. This is especially evident since the front face of PAD B is intended to be planar. This washover reduces the "residence time" of slurry on the face of PAD B.
A silicon wafer 120 is lightly pressed (flat) against the front surface of PAD B so that formations (on the pad-confronting face of the wafer) sought to be polished are acted upon by the action of PAD B and the slurry. Typically, the pads 102 and 108 and the platen 106 are on the order of 20-30 inches in diameter, the wafer is 4-6 inches in diameter, and polishing is performed in the center 2/3 (two-thirds) portion (area) of PAD B.
As the slurry is used, it exits the front surface of PAD B and, as noted above, is not recovered. Evidently, the slurry must be fed onto PAD B at the rate that it exits PAD B, and preferably the rate would be optimized so that the slurry is entirely used, with no loss of un-depleted (un-consumed) slurry. However, this is generally not the case, and the slurry feed rate is established to be higher than would be necessary to deplete the slurry. The slurry is illustrated in FIG. 1 as a single layer of dots, indicating that it is difficult to retain slurry on the front face of PAD B.
Typical pad materials are: (1) for PAD A, foamed polyurethane; and (2) for PAD B, polyester felt stiffened with polyurethane resin. The adhesive backings 104 and 110 for the pads are typically polyurethane based. Generally, it is preferable that PAD B is stiffer than PAD A. In the case that both pads are doped with polyurethane resin, this can be achieved simply by doping PAD B with more polyurethane than PAD A.
Two failure modes are of particular interest. In one mode, the chemicals from the slurry are gradually "wicked" into the pads and gradually attack the adhesive, and adhesive failure can be expected in about three days, which is generally acceptable. This is illustrated at 122, where slurry is shown permeating PAD B and attacking adhesive 110. In another, catastrophic mode, the slurry attacks the adhesive bond between PAD A and PAD B directly, edgewise at the adhesive boundary between the pads, and failure may occur within one half hour, which is very unacceptable. This is illustrated at 124, where slurry is shown attacking the adhesive 110 edgewise between the pads. Eventually, and usually abruptly, the pads will delaminate (come unglued) from one another, which will require stopping the polishing process, and re-setting up the polishing equipment. This is not desirable.
It is known to provide a plastic ring (dam) around the pads, extending upward above the front face of PAD B, primarily for the purpose of creating a reservoir of slurry on the front face of PAD B, which will allow the slurry to be retained longer on the face of PAD B. This is one approach to increasing "residence time" and optimizing the chemical depletion of the slurry before it overflows the ring. However, using a plastic ring required additional setup, and is of little effect with regard to the adhesive failure modes discussed above.